When running a tool in a casing downhole, it may often be desirable to know the specific position of the tool. Especially when running different types of inspection or logging tools, it may be very important to know the exact location of each observation made during the inspection or logging. For this and several other reasons, many attempts have been made to develop a device capable of determining the position of the tool when situated downhole.
A known device for determining the position of a tool downhole is called a “Casing collar locator”. Typically, a locator comprises one or more magnets and one or more coils for measuring changes in the magnetic and electrical flux when passing a collar or casing feature significant enough to cause a measurable change. The device is thus only able to determine the position of the tool collar by collar and not between two collars, and in order to measure differences in the flux, the device must be moved with considerable speed in the casing.
Another “Casing collar locator” comprises one or more flux gate magnetometers in the form of coils for measuring the remaining or naturally occurring magnetism in the casing collars. However, in order to determine the position of the tool between the collars, the wireline depth must be used.
As shown in U.S. Pat. No. 6,768,299, a “Casing collar locator” may also comprise one magnetometer for measuring changes in the strength of the magnetic field generated from a magnet. The “Casing collar locator” may also comprise more than one magnetometer to achieve different modes of detection so that one magnetometer detects collars and the other magnetometers detect the direction of the tool in order to control the direction of the drilling head of the tool.